Two mechanisms served by a single transition state: The branching ratio for CH2OĽ» + CH3Cl on improved potential energy surfaces

COMP 201

Jie Li, jli@chem.wayne.edu1, Xiaosong Li, xiaosong.li@yale.edu2, Sason Shaik, sason@yfaat.ch.huji.ac.il3, and H. Bernhard Schlegel, hbs@chem.wayne.edu1. (1) Department of Chemistry, WAYNE STATE UNIVERSITY, 5101 Cass Ave., Detroit, MI 48202, (2) Department of Chemistry, Yale University, P.O. 208107, New Haven, CT 06520, (3) the department of organic chemistry and the lise meitner-minerva center of computational chemistry, Hebrew University, Jerusalem 91904, Israel
The reaction of a formaldehyde radical anion with methyl chloride has been a celebrating example in which a single transition state leads two products: substitution at carbon (Sub(C)) and electron transfer (ET). Ab initio molecular dynamics has been used to study this reaction at UHF/6-31G(d), BHandHLYP/6-31G(d) and other levels of theory. There are ca. 200 trajectories for each level of theory integrated with initial conditions sampled from thermal distribution at 298K at the transition state. Three types of trajectories are studied in detail: trajectories go directly to ET product, trajectories go to Sub(C) product, and trajectories initially go to Sub(C) valley then dissociate to ET products in the end. Energetics and dynamics are compared at different levels of theory. It was found that the branching ratio of Sub(C) to ET is highly dependent on the shape of the potential energy surface. Therefore it is important to use a level of theory for the ab initio molecular dynamics that adequately reproduces the energetics obtained from higher accuracy calculations.